1. Field of the Invention
The present invention relates to an image fixing apparatus for fixing a toner image to a record medium and a method of controlling the same.
2. Description of the Related Art
As is well-known in the art, an electrophotographic image forming apparatus, such as a laser beam printer, an LED printer, a digital copying machine, a facsimile and the like, is an appliance for transferring image signals to a record medium as a visual image according to digital signals inputted from a computer or a scanner.
Specifically referring to the operation of such an image forming apparatus, image signals form an image on a photoconductor medium in a form of an electrostatic latent image. The electrostatic latent image formed on the photoconductor medium is visualized as a visual image by toner. The visual image is transferred to a record medium, such as print paper, OHP (overhead projection) film, envelopes, and the like. Then, as the record medium to which the visual image is transferred passes an image fixing apparatus, the visual image is fixed to the record medium, which is in turn discharged out of the image forming apparatus.
Meanwhile, because the toner image that is transferred to the record medium is deposited to the record medium by a weak electrostatic force, it is apt to scatter. The image fixing apparatus is means for applying predetermined heat and pressure to the record medium to which a toner image is transferred, thereby melting and fusing the toner of the image to the record medium. The image, which has been subjected to such an image fixing process, can be substantially permanently preserved.
Generally, the above-mentioned image fixing apparatus includes a heat roller provided with a heat source, such as a heat lamp, in the inside thereof to transfer the heat generated from the heat source to a record medium, and a pressure roller for compressing the record medium against the heat roller. The heat roller has a cylindrical surface formed of a metallic material on which surface a Teflon resin is coated. The pressure roller has a surface formed of a soft material, such as rubber or the like, and compresses the record medium through compression means, such as springs, which are mounted at the opposite ends of the pressure roller.
Such an image fixing apparatus should meet the following requirements:
i) the fixing of an image shall be easily executed within a proper range of temperature of the heat generated from the heat roller; and
ii) a wrap jam phenomenon, which causes a toner image on a record medium to be deposited to the heat roller as the record medium is curled under the influence of the heat from the heat roller, and wrinkles, crumples or the like of the record medium, which may be produced as the record medium is heated, shall be minimized.
In many cases, however, the above-mentioned requirements may be in conflict with each other. For example, if the temperature of the heat roller is increased to enhance the fixability of a toner image, a record medium is adhered to the heat roller, whereby wrap jam, wrinkles, crumples, and so forth, are frequently produced. To the contrary, if the temperature of the heat roller is reduced, the fixability of the toner image is deteriorated. Additionally, if the compressive force of the pressure roller for compressing the record medium against the heat roller is increased, the length of time of transferring heat from the heat roller to the record medium is also increased, whereby the fixability of the toner image may be enhanced. However, the endurance of the pressure roller may be deteriorated.
Recently, to efficiently prevent wrap jam and to improve the fixability of toner images in an image fixing apparatuses having features that are in conflict with each other as described above, an arrangement in which plural pressure rollers come into rotational contact with a heat roller, has been widely adopted.
FIG. 1 shows an image fixing apparatus with such an arrangement employing a pair of pressure rollers, by way of an example, wherein the image fixing apparatus includes a heat roller 10 having a heat source 11 within the inside thereof. First and second pressure rollers 21 and 22 rotationally contact the surface of the heat roller 10.
The heat roller 10 is coated with a sheath 12 of a resilient material over the peripheral surface thereof. For example, the sheath 12 may be formed from a silicon polymer, an elastomer or the like. The sheath 12 may be coated with a Teflon coating layer, so that a toner image can be more easily released.
The first and second pressure rollers 21 and 22 are mounted in such a manner as to be rotatable about rotary shafts 21a and 22a, respectively. Each of the opposite ends of the rotary shafts 21a and 21b is rotatably supported by a bushing member 26, which is in turn biased toward the heat roller by compression means 25, such as a coil spring.
The sheath of the second pressure roller 22 is formed from a material that is harder than the sheath 12 of the heat roller 10, so that the sheath 12 of the heat roller 10 is deformed at the area where the heat roller 10 and the second pressure roller 22 rotationally contact each other. The peripheral surface of the first pressure roller 21 is formed from a material that is softer than the sheath 12 of the heat roller 10, so that the first pressure roller 21 is deformed at the area where the heat roller 10 and the first pressure roller 21 rotationally contact each other.
A first image fixing nip is created at the area where the heat roller 10 and the first pressure roller 21 rotationally contact each other, and a second image fixing nip is created at the area where the heat roller 10 and the second pressure roller 22 rotationally contact each other.
After an image is fixed to a record medium P at the first and second nips by the heat roller 10 and the first and second pressure rollers 21 and 22 and the record medium P is curled over the heat roller 10 at the first nip under the influence of the heat of the heat roller 10, the record medium P is compressed in the opposite direction at the second image fixing nip by the second pressure roller 22, whereby the record medium P is discharged in a gently flattened state. That is, the record medium P with an image being fixed is curled in the direction opposite to the heat roller 10 by the compressive force applied by the second pressure roller 22, whereby the record medium P is discharged in the flattened state through the image fixing apparatus by the reverse curling.
Additionally, although the toner image on the record medium P may be deposited to the heat roller 10 side by the first pressure roller 21 of the soft material while the record medium is passing the first image fixing nip, the toner image may be smoothly released from the sheath 12 of the heat roller 10 because the sheath 12 of the heat roller 10 is abruptly deformed due to the hard characteristic of material of the second pressure roller 22, as well as because the record medium P is reversely curled by the compressive force applied by the second pressure roller 22 while the record medium P is passing the second image fixing nip, whereby the wrap jam phenomenon is prevented.
As described above, an image fixing apparatus, which employs plural pressure rollers, has advantages of not only improving the fixability of a toner image due to the increase of the compressive forces of the pressure rollers, but also preventing wrap jam, crumples, and wrinkles.
However, the above-mentioned prior art has a limit in that because the magnitudes and/or the directions of the compressive forces to be applied by the first and second pressure rollers 21 and 22 are specified and fixed in such a manner as to be suitable for ordinary print papers, it is extremely difficult to properly cope with parameters of a certain record medium (for example, thickness, quality of material, and any other characteristic of the record medium), density of a toner image to be deposited to the record medium, or the like.
Accordingly, a need exists for an image fixing apparatus having a compression unit in which the compressive forces applied by a plurality of pressure rollers are adjustable.